a laboratory study on cold-mix cold-lay emulsion mixtures (2)
DESCRIPTION
study of cold asphalt mix designTRANSCRIPT
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I. N. A. Thanaya
Lecturer, Civil Engineering
Department, Udayana University,
Denpasar-Bali, Indonesia
S. E. Zoorob
Private consultant, formerly
Lecturer, School of Civil Engineering,
University of Leeds, UK
J. P. Forth
Senior Lecturer, School of
Civil Engineering, University
of Leeds, UK
Proceedings of the Institution of
Civil Engineers
Transport 162
February 2009 Issue TR1
Pages 4755
doi: 10.1680/tran.2009.162.1.47
Paper 800021
Received 21/12/2005
Accepted 16/05/2007
Keywords:
pavement design/reuse & recycling
of materials/strength & testing of
materials
A laboratory study on cold-mix, cold-lay emulsion mixtures
I. N. A. Thanaya Beng, PhD, S. E. Zoorob MEng, PhD and J. P. Forth BEng, PhD, MASCE
This paper describes laboratory experiments and presents
results for the performances of cold-mix, cold-lay
emulsion mixtures. The main objective of the experiments
was to evaluate and improve the properties of the cold
mixtures. The mixture properties evaluated were:
volumetric properties, indirect tensile stiffness modulus
(ITSM), repeated load axial creep and fatigue. These
properties were compared with conventional hot asphalt
mixtures not containing any waste/recycled materials. To
optimise the performances of the mixtures, a target of
ITSM value of 2000MPa was selected. At full curing
conditions, the stiffness of the cold mixes was found to be
very similar to that of hot mixtures of the same
penetration grade base bitumen (100 pen). Test results
also show that the addition of 12% cement significantly
improved the mechanical performance of the mixes and
significantly accelerated their strength gain. The fatigue
behaviour of the cold mixes that incorporated cement was
comparable with that of the hot mixtures.
1. INTRODUCTION
Until now, cold mixes have been considered inferior to hot
asphalt mixtures. There are three main concerns
(a) high air-void content of the compacted mixtures
(b) weak early life strength (caused mainly by the trapped water)
(c) the long curing times (evaporation of water/volatiles
content and setting of the emulsion) required to achieve
maximum performance.
Studies by Chevron Research Company in California concluded
that full curing of cold bituminous mixtures on site may occur
between 2 and 24 months depending on the weather
conditions.1
In the UK, the publication of Specication for Reinstatement of
Openings in Highways2 by the Highway Authority and Utility
Committee (HAUC) in 1992 helped to draw attention to cold-
mix, cold-lay materials (cold mixes). The report allows the use
of permanent cold-lay surfacing materials as an alternative to
hot-mix asphalts for reinstatement work on low-volume roads
and footways. A principal requirement is that these mixes
perform adequately for a guaranteed period of 2 years, which
has proved a challenge.1
In general, cold mixes are simple to produce and suitable for
low-to-medium trafc conditions, work in remote areas and for
small-scale jobs (at numerous locations) such as reinstatement
work. However, currently there is no universally accepted mix
design method. Furthermore, due to a lack of uniformity in the
laboratory procedures for cold-mix curing and evaluation of
mechanical properties, in particular at an early age, it is difcult
to form reliable correlations between the experimental results
reported in the literature by different researchers.
The cold-mix samples were produced using a simplied (more
practical) method previously proposed by the authors.3,4 This
paper presents an investigation into the volumetric properties
and mechanical performance of the cold mixes, the effect of
storage time on loose cold mixes, and a realistic evaluation of
the rate of strength gain of cold mixes cured outdoors. For this
investigation, the target compacted mix air void content was
510%. This air voids target is consistent with the most recent
amendment to the HAUC document, which requires that the air
voids after compaction of cold mixes used for carriageways is
between 2 and 10%. It also agrees with guidance provided
within the MPW-RI specication.5 A target minimum indirect
tensile stiffness modulus (ITSM) of 2000MPa was also a
requirement.2,6
The investigation used carboniferous limestone as the coarse
aggregate. This is widely available in the UK. A by-product of
red porphyry stone crushing was used as the ne aggregate.
This red porphyry sand (RPS) is produced during the production
of decorative red-coloured coarse aggregates that are commonly
used for chipping applications. The RPS availability uctuates
depending on the demand for the coloured coarse aggregates.
Currently RPS is mainly used as ll material and for sub-grade
formation.
Fly ash was used as a ller material. Traditionally, y ashes
have been used in a range of applications, namely as ll
materials, for grouting, and soil stabilisation. Fly ash has also
been used in road pavements as road bases, sub-bases and for
sub-grade formation.7
From a pavement engineering perspective, the most useful types
of mixtures that utilise y ash for road pavements are y ash-
bound mixtures and granular y ashes. These mixtures rely on
the pozzolanic properties of the y ash, which can harden in the
presence of water and lime and/or cement. Fly ash is also used
in cement-bound mixtures (building blocks, concrete, etc.).7
Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al. 47
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Fly ash has also been used as
a replacement for
conventional limestone llers
in hot rolled asphalts.8 As y
ash particles tend to be more
spherical in shape when
compared with limestone,
laboratory investigations have
shown that y ashes can
enhance the workability of the
bituminous mixtures, which is
of great benet particularly
during the compaction process. Based on the laboratory results
and as part of a resurfacing programme on the A689, UK in
April 1991, a full-scale 320m length trial section consisting of
several hot-rolled asphalt segments containing pulverised y
ash ller was laid and compacted. Subsequently, the road
sections were continuously monitored for any signs of distress
over a period of 5 years and all trial sections were shown to
perform [as expected] adequately.8 Based on the success of the
project, a publication was issued by the energy efciency ofce
of the Department of the Environment, on Future Practice (R&D
prole 52) carrying the title The Use of Pulverised Fuel Ash as a
Bituminous Filler.9
The utilisation of y ash in the UK has remained stable for a
number of years (at around 50% of production). The amount of
y ash available in stock, in addition to fresh production is
estimated to have remained relatively constant at about
250 000 000 t for a number of years.7
The mixtures also incorporated crushed glass. In the UK, the
glass manufacturing sector attempts to operate a closed-loop
recycling system; however, it has a limited capacity to accept
green and mixed colour glass. As glass collection increases (to
meet the 2006 packaging targets of 60%)10 an excess
(300 000400 000 t) of green glass is likely, for which alternative
high-value, high-volume markets are required.
The incorporation of crushed glass, in particular green/mixed
glass into bituminous mixtures is not new. Glasphalt hot
mixtures incorporating 30% crushed glass (using a 100 pen.
bitumen), laid at a trial site in Milton Keynes by RMC
Aggregates Ltd. showed an average ITSM value of 1900MPa.
Meanwhile, in the same trial, the control hot mixture (not
containing crushed glass) gave 2200MPa. The average air-void
contents for the Glasphalt and the control mixtures on that trial
were 4.9 and 4.7%, respectively.11
2. AGGREGATE GRADATIONS FOR THE MIXTURES
The aggregate gradations of all the design mixtures used in this
investigation were based on a modied Fullers curve using a
formula proposed by Cooper et al.12 as shown below:
P 100 F dn 0:075n
Dn 0:075n F1
where P is the percentage of material passing sieve size d (mm);
D is the maximum aggregate size (mm); F is the percentage
ller; and n is an exponential value that dictates the concavity
of the gradation line. The value used for n was 0.45, which
provided good aggregate packing. A value of 4% was selected
for F, which satised the allowable limits for ller content in
dense bitumen macadams.13
3. MATERIALS USED
The constituent materials used for the cold mixes and their
properties are given in Table 1.
The binder used was a cationic bitumen emulsion (60% bitumen
content) with 100 pen. grade base bitumen obtained from Nynas
Bitumens. The specic gravity of the base bitumen was 1.02.
The maximum aggregate size selected for all the cold mixes was
12mm. The gradation of the cold mixes is plotted in Figure 1.
This gradation is within the limits recommended by the
American Asphalt Institute.14 The optimum residual bitumen
content of the cold mix was previously designed at 6%.
4. MIXTURE DESIGNATION
Depending on the material types incorporated, the cold mixes
were designated as shown in Table 2.
The coarse aggregates for all mixtures were composed of the
following fractions (1210mm), (105mm), (52.36mm), and
the ne aggregates were of particle sizes (2.360.075mm). All
ller material passed 0.075mm.
As shown in Table 2, three types of cold mix were produced.
Initially cold-mix-0 mixtures were made, but it was found that
some aggregate particles of the cold-mix-0 mixtures were not
fully coated with the bitumen emulsion. Although cationic
bitumen emulsion was recommended to be compatible with
most aggregates,1 this was certainly found not to be the case
Material Density: g/cm3 Water abs.: % Usage
Limestone 2.65 1.0 Coarse aggregateCrushed glass 2.51
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with the RPS used in this investigation. This has been
previously explained as a result of the afnity between bitumen
emulsion and aggregates, which can be affected by the surface
charges of the materials.1
In order to improve the degree of coating in the cold-mix-1
mixtures, the ne aggregates portion (as shown in Table 2) were
made up of a combination of a maximum of 60% RPS and 40%
quartzite sand. This composition was based on coating test
trials.
Cold mix-2 incorporated 30% crushed glass by weight of
total aggregates and was composed of 60% 52.36mm
particles and 40% passing 2.36mm. This was based on the
availability of the crushed glass particle sizes. Replacement
levels of up to 30% by mass of aggregates in hot mixtures
(Glasphalt) have already been successfully tested in full-scale
trials in the UK with encouraging performance results.11 The
performance of Glasphalt was also compared with that of
cold mix-2.
In cases where an accelerated rate of strength gain was required,
1 to 2% cement by mass was added to the mix to accelerate the
curing process. Rapid-setting cement was found to further
accelerate the rate of strength gain.15
5. MIXTURE PRODUCTION AND MECHANICAL
PROPERTIES TESTED
Currently, there is no universally accepted design mixture for a
cold mix. In this investigation the mixtures were produced using
a simplied cold-mix design procedure developed previously by
the authors.3,4 This design procedure overcomes the impractical
requirement to determine the optimum water content at the
compaction on site. It ensures that the air void requirement is
met; that the retained stability is evaluated at optimum residual
bitumen content only, which reduces the number of samples
needed, and it also provides data on the ultimate strength of the
cold mix under full curing conditions.
The mix design procedure involves a number of sequential steps
which are as follows
(a) determination of aggregate gradation
(b) estimation of initial emulsion content
(c) binder coating tests with variations in pre-wetting water
content to obtain the best aggregate coating
(d ) determination of adequate compaction level to satisfy nal
air-void requirements
(e) variation of emulsion content
( f ) curing of compacted specimens
(g) determination of optimum residual bitumen content
(h) determination of retained stability at optimum residual
bitumen content
( j ) determination of ultimate strength at full curing conditions.
In summary, the mixing process was carried out as follows:
3600 g of aggregate material, which had been proportioned,
were prepared (for three samples). The aggregates were dry
mixed, then combined with the required pre-wetting water to
produce an even mix. The required bitumen emulsion was then
added to the aggregates and mixed until a satisfactory coating
was obtained. Before compaction the mixture was neither too
sloppy nor too stiff. Air drying may be needed if the mixture
becomes too sloppy.
Compaction of the cold mix specimens was carried out using a
Gyropac16 compactor set at 240 kPa axial pressure (for 100mm
diameter samples) with an angle of gyration of 28. Two
compaction levels are routinely used with this type of
equipment.
(a) Medium compaction level: 80 revolutions in the Gyropac,
which is equivalent to the compaction effort generated
when applying 50 blows to each end of the sample using a
Marshall hammer.
(b) Heavy compaction level: 120 revolutions in the Gyropac,
which is equivalent to the compaction effort generated
when applying 75 blows to each end using a Marshall
hammer.
Following several compaction trials with the Gyropac, it was
found that in order to achieve the target air-void content of 5
10% the compaction level had to be increased by up to twice
the heavy compaction level routinely specied. This higher
compaction level was subsequently referred to as extra heavy
compaction.
Laboratory curing of the cold mix was carried out in an oven
set at 408C. Full curing conditions were achieved when the
specimens, following repeated weighing, maintained a constant
mass at 408C. Typically, full curing conditions were achieved
within 1821 days for samples with air void values in the range
89%.15
The mechanical properties tested were: ITSM17 at 208C, dynamic
creep at 408C and fatigue at 208C.18 A universal materials
testing apparatus (MATTA) was used for these tests.
6. ITSM COMPARISON OF THE COLD MIX TO HOT
MIXTURES (100PEN. GRADE)
Details of the ITSM tests are shown in Figure 2. The ITSM of the
various cold mixtures were compared with hot mixtures
produced with the same 6% bitumen content. The bitumen was
Mixture type Coarse aggregates (56.6%) Fine aggregates (39.4%) Filler (4%)
Cold mix-0 Limestone Red porphyry sand Fly ashCold mix-1 Limestone 60% red porphyry sand
40% quartzite sandFly ash
Cold mix-2 68% limestone32% crushed glass
70% Quartzite sand30% crushed glass
Fly ash
Table 2. Mixture designation and composition of the cold mix
Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al. 49
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an equivalent grade to the base bitumen used for the emulsion
(that is, 100 pen.). Two hot mixtures were produced from
limestone coarse aggregate, ne quartzite sand and y ash
ller. The mixes were graded identically, however, the mixing
and compaction temperatures were set at 140 and 1258C,
respectively.19 The mixtures were compacted using a Gyropac
compactor. One of the mixtures was compacted using a medium
compaction effort, whereas the other hot mix was compacted
using heavy compaction. The properties of the mixtures are
given in Table 3.
Referring to Table 3, in order to meet the target air void range
of 5 to 10%, the cold mix was compacted twice at the heavy
compaction level. Even subjected to this degree of compaction,
air void values in the range of 8 to 9% could only be achieved.
It is well known that high air void content is an inherent
problem of cold mixes.1 This is because the bitumen emulsion
partially sets during compaction at the ambient temperatures.
This causes the mixture to stiffen and leads to higher amounts
of air voids when compared with hot mixtures.
The need for a heavier compaction level for cold mixes is well
known within the highways industry.1 These tests were
laboratory tests in which the application of a heavy compaction
level using the Gyropac was easily achievable. On site, heavier
compaction efforts can be achieved using a heavier road roller
and a greater number of passings; however, this raises issues
of efciency. Furthermore, over-compaction of the lower
pavement layers needs to be carefully monitored. Nonetheless,
successful cold mix site trials have been achieved.20 The
successful introduction of cold mixes into practice also depends
on the manufacturer of the bitumen emulsion. The pavement
industry requires a bitumen emulsion with improved workability
allowing the mixtures to be compacted at lower compaction
levels. This is not beyond the capability of the manufacturers.
Laboratory experiments which prepared the test samples in two
thin layers of cold mix have previously been carried out by the
authors. The second layer was applied after the rst layer had
undergone some degree of curing. A priming coat was applied
to the surface of the rst thin layer, before applying the second
layer. It was found that the performance of these layered
samples was satisfactory.15 The application of thinner double
layers on site would require less compaction.
Full curing conditions (in an oven set at 408C) were achieved
after 1821 days. The stiffness of most of the cold mix
specimens, subjected to full curing conditions, did meet the
minimum stiffness target of 2000MPa, with the exception of
cold mix-0. This was thought to be due to reduced coating
achieved in this mix as explained in Section 4. Cold mix-1 and
cold mix-2 gave better coating than cold mix-0 and hence gave
higher ITSM values. The addition of 1 to 2% cement
signicantly increased the ITSM values.
The air void of cold mix-2 (incorporating 30% crushed glass)
was slightly lower than that of cold mix-1. This is because
crushed glass offers less friction during compaction. These
results are comparable with the results obtained from hot
Glasphalt mixtures11 mentioned earlier.
The results shown in Table 3 also show that the air voids of cold
mix-2 was somewhat higher than the Glasphalt hot mixtures.
On the other hand, the ITSM values of cold mix with 30%
crushed glass at full curing were comparable (somewhat stiffer)
than Glasphalt, even without the incorporation of cement. Due
to its amorphous nature and its high silica content, when nely
ground, glass possesses pozzolanic properties.20 The crushed
glass used within this investigation contained about 0.85%
passing 0.075mm (ller fraction). This might have contributed
to the improved strength of the cold mix incorporating crushed
glass.
In comparison with cold mixes, hot bituminous mixtures [when]
mixed and compacted at the correct temperatures normally
possess better coating and workability properties as well as
lower air void values at lower compaction efforts. This is
evident from the air voids and compaction effort results shown
in Table 3.
When cold mixes were brought to full curing condition by oven
drying at 408C for 1821 days (until achieving a constant
weight), the samples should have undergone some level of
oxidation, hence stiffening the bitumen.15
Figure 2. ITSM test configuration
Mixture type Compactioneffort
Air voids:%
ITSM:MPa
Cold mix-0 2 heavy 9.7 1680Cold mix-0 2% RSC 2 heavy 9.6 2500Cold mix-1 2 heavy 9.6 2276Cold mix-1 1% RSC 2 heavy 9.3 3378Cold mix-1 2% RSC 2 heavy 9.2 4973Cold mix-2 2 heavy 8.4 2275Cold mix-2 1% RSC 2 heavy 8.6 3290Cold mix-2 2% RSC 2 heavy 8.8 4339100 pen. hot mix Medium
compaction4.7 2161
100 pen. hot mix Heavycompaction
3.4 2525
RSC, rapid setting cement.
Table 3. Properties of the cold mix compared to hot mixes withsame binder grade (100 pen.)
50 Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al.
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7. EFFECT OF STORAGE TIME ON COLD MIXES
This part of the investigation was to evaluate the effect of
storage time of the loose cold mix following mixing and prior
to compaction. The length of storage time required is a variable
and depends on the scale of the proposed works. In practice, it
is important to be able to store loose cold mix for small-scale
works, such as road maintenance operations performed at
various locations along a section of road.
It has been mentioned in the previous section that the
performance of cold mix is inuenced by the afnity between
the aggregate and the bitumen emulsion.1 This was the case
with the cold mix-0 mixtures (Table 2) that were used for this
particular experiment. A slightly lower ITSM value was
therefore recorded (Table 3) due to the reduction in coating.
The cold-mix, wet-loose mixtures were stored (following mixing
with the emulsion and after lightly air drying the mixture to
avoid clumping) in a sealed container for 0, 3, 6, 24, and 48 h
before compaction. The compacted samples were then fully
cured in an oven at 408C (until a constant mass was achieved).
Two types of mixtures were tested: mixtures without cement
and mixtures with 2% rapid setting cement (RSC). Test results of
air voids and ITSM values at 208C of the fully cured compacted
samples are presented in Figures 3 and 4. As the loose mixtures
were lightly air dried before they were stored in a sealed
container, very little water was squeezed out during the
compaction process.
Figure 3 illustrates that as the mixtures became stiffer (and
therefore less workable) during compaction in line with storage
time, there was an increase in air voids. Cold mix-0 containing
RSC was less workable than without RSC. This was indicated by
an increase in air void content.
As the emulsion sets with increasing curing time, this leads to an
increase in compacted air voids with increased storage time. Cold
mix-0 2% RSC had a larger number of air voids than the loose
mixture for the entire storage duration with a smaller difference
recorded within the rst 6 h. These higher values are due to the
hydration of cement in the presence of moisture. This causes the
loose mixture to be somewhat stiffer and less workable during
compaction when compared with the mixture without cement.
Figure 4 shows that the ITSM values of cold-mix-0 mixture
(fully cured with no cement) remain relatively unchanged up to
24 h storage time with a very gradual reduction beyond 1 day
storage. The ITSM values of the fully cured cold mix-0 2%
RSC on the other hand were overall greater than the mixture
without cement at all storage times. However, there was a very
noticeable reduction in ITSM values within the 3 and 6 h loose
mixture storage times.
It is interesting to note that the highest ITSM value for the
mixture with cement was obtained when the wet mixture was
compacted immediately following the mixing process (time 0 h).
This matter was not fully investigated; however, it is discussed
below.
The results suggest that when the mixtures were compacted
within a very short time of the cement being added, the cement
hydration products would have been allowed to form in the
available spaces between the mineral aggregates and hence the
hydration products act as bonds between the aggregate
particles. Simultaneously, they form wedges of hydrated
cement-nes mortar which lock the aggregate skeleton in place.
Due to hydration, the crystalline cementitious products form
mainly in the rst 6 h. Therefore, any delay in compaction from
(time 0) will allow more hydration products to form in the loose
wet mixture. This delayed compaction during the rst few hours
(up to 6 h in this case) would have caused damage to these
relatively weak crystalline formations, which results in a
reduction in bond between the mineral aggregate particles and
hence lower ultimate ITSM. (It is similar to having increased the
ne aggregate content of the mixture.)
After the 24 and 48 h loose mixture storage times, the majority
of the cement hydration process would have taken place and the
cementitious mortar formed would have become hard or tough
enough so that any damage to the bonds between the crystalline
phase and the mineral aggregates during compaction should
have greatly reduced. As the hardened cement phase occurs (as
irregularly shaped wedges) in the interstices between the larger
mineral aggregate particles, it is logical that when these mortar
wedges were partially broken into medium-sized particles, the
increased roughness of the mixture (which was evident from the
increased specimen air voids) provides an improvement in the
707580859095
100105110115120
0 5 10 15 20 25 30 35 40 45 50Loose mix storing time: h
Aver
age
poro
sity:
%
Cold mix-0Cold mix-0 +2%RSC
Figure 3. Air voids of fully cured compacted CBEM-0 plottedagainst loose mixture storage time
1300
1500
1700
1900
2100
2300
2500
2700
0 5 10 15 20 25 30 35 40 45 50Loose mix storing time: h
Aver
age
ITSM
: MPa
Cold mix-0Cold mix-0 +2%RSC
Figure 4. ITSM of fully cured compacted cold mix-0 plotted againstloose mixture storage time
Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al. 51
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aggregate interlock. Furthermore, the higher angle of internal
friction that is present in these mixes due to the hydrated
cement particles results in greater values of ITSM.
8. OUTDOOR CURING OF THE COLD MIXES
Samples of cold mix-1 mixes, without and with 2% RSC were
manufactured and cured under out-door exposure conditions
(full exposure to rain and actual temperature proles) in order
to better simulate on-site conditions. This was carried out to
evaluate the mixtures realistic rate of strength gain with time.
Following compaction, the samples were kept in their moulds
for 24 h at 248C room temperature prior to extrusion. The sides
of the samples were then sealed with plastic adhesive tape and
placed on a at metallic surface outdoors (on the roof of the
school of civil engineering building, University of Leeds), as
shown in Figure 5. The objective of this treatment was to
simulate realistic site curing conditions where the evaporation
of water will predominantly occur through the surface of the
mixture.
Outdoor curing began in February 2002, with an average
outdoor temperature of 108C. Rain, which was a very frequent
occurrence during the specimens curing period, resulted in a
slight wear of the surfaces of the samples. However, the samples
remained intact and in good shape. At regular intervals, two
samples were selected for testing. Contrary to British Standard
recommendations, each sample was tested for ITSM at 208C
only once, in order to avoid damage (as had been experienced )
when a sample was subsequently tested at a later age. The
samples were tested at ages: 1, 2, 4, 6, 8, 10, 12 and 24 weeks.
The rate of strength gain (in terms of ITSM at 208C) of the
samples is shown in Figure 6.
With respect to outdoor curing, samples with no added cement
gained strength slowly with time (as shown in Figure 6). The
ITSM target of 2000MPa2,6 was only achieved after an
estimated 16 weeks of outdoor curing. On the other hand,
samples with 2% RSC showed greatly improved performance, as
they required less than 2 weeks of curing to meet the ITSM
target of 2000MPa. Stiffness values were not tested beyond 24
weeks (6 months), but the trend shown in Figure 6 does indicate
a much more gradual but continued gain in stiffness. In either
case, the rate of gain of strength was relatively fast when
compared with some cold mix site trials without cement which
required much longer curing times of 2 to 24 months.1 Direct
comparison with other full-scale cold mix surfacing
investigations was not possible as the mixtures in the eld were
inuenced by various factors, including; compaction level,
climatic conditions: rainfall, surface drainage, trafc conditions,
etc.
9. CREEP PERFORMANCE
Dynamic creep tests were carried out using a universal material
testing apparatus (MATTA) as shown in Figure 7 (a typical
conguration for illustration), and was set at the following test
conditions
(a) pulse width, 1000ms
(b) pulse period, 2000ms
(c) test termination strain, 100 000 ms
(d ) terminal pulse count, 3600 pulses
(e) conditioning stress, 10 kPa
( f ) loading stress, 100 kPa1922
(g) time, 15min
(h) pre-load rest time, 2min
( j ) recovery time, 2min.
The tests were carried out at 408C. In order to reduce friction on
the end plates, the top and bottom of the samples were dipped
in silicon powder. The sizes of the samples were around:
Figure 5. Outdoor curing of cold-mix samples
Figure 7. The configuration of the repeated load axial creep test
1000
0
2000
3000
4000
5000
6000
0 2 4 6 8 10 12 14 16 18 20 22 24 26Outdoor curing time: weeks
ITSM
(stiff
ness
): MPa
Cold mix-1Cold mix-1 +2%RSC
Figure 6. Rate of strength gain of cold-mix samples curedoutdoors
52 Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al.
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6265mm and 100mm in diameter. This size of samples had
been commonly used by researchers in Leeds8,23 and is
consistent with the British Standard Draft for Development DD-
185: 1990.24
The creep performance of the cold mixes (number of load cycles
plotted against cumulative strain) is shown in Figure 8. As is
generally the case for creep behaviour, the creep curve can be
divided into three main regions; the primary creep region where
the strain rate decreases with the number of load cycles applied;
the secondary creep region where the strain rate is almost
constant, otherwise known as the steady-state strain rate; and
the tertiary creep region where the strain rate increases rapidly
up to failure.
The steady-state strain rates, or creep slope values are shown in
Table 4. The results are
compared with readily
available data from hot
mixtures.23
Referring to the creep test
results in Figure 8, in general,
the creep of the mixtures is
directly related to their
stiffness (ITSM) values.
Naturally, a higher ITSM value
leads to a higher creep
stiffness value, which in turn
leads to reduced permanent
deformation and lower creep
slope values.
It is interesting that in Figure
8, the cold mix-1 2% RSC
mixes were found to have
lower creep deformations (i.e.
better performance) in
comparison with the hot
bituminous mixtures made
with harder 50 pen. bitumen.
Without cement, however, the
cold mix-1 gave a lot higher
deformation than the hot mixtures. Referring to Table 5, the cold
mixes are clearly suitable for low to medium trafcked roads.16
10. FATIGUE PERFORMANCE
Fatigue is a phenomenon of fracture or failure under repeated
loading. Fatigue test results can be expressed as a relationship
between the number of loading cycles, Nf (pulses) to strain (e) at
failure.18 The fatigue testing conguration is shown in Figure 9.
The fatigue performances of the cold mixes were compared with
14mm maximum aggregate size asphalt concrete mixes of
100 pen. asphalt with 5% air void25 with 5% optimum asphalt
content. The fatigue lines are shown in Figure 10 (in a loglog
scale).
The fatigue line equations and correlation coefcient R2 from
the fatigue tests are presented in Table 6.
2000
0
4000
6000
8000
10 000
12 000
14 000
0 1000 2000 3000 4000Load cycles: pulses
Axia
l stra
ins:
mic
rost
rain
s
Cold mix-1Cold mix-1 +2% RSCAC-50 penHRA-50 pen
Figure 8. Cold mixes dynamic creep test results (strain plottedagainst load cycles) at 408C, compared with hot mixes
Mixture Testtemp.:8C
Linear regression
equations and R2 Slope of
creep curve:e/pulse
Cold mix-1 40 y 1:42x 6963:1 1.42Cold mix-1 2% RSC 40 y 0:02x 2956:1 0.02Asphalt concrete 40 y 0:3378x 5614:1 0.338Hot rolled asphalt 40 y 0:4229x 6041:1 0.423
Equations applicable to the range from 12003600 pulses (see Figure 9).R2 values for all equations were higher than 0.95.
Table 4. Cold mix dynamic creep slope data (referring to Figure 8)
Average annualpavementtemperature: 8C
Heavy traffic:>106 ESA
Medium traffic:5 105 to 106 ESA
Light traffic:30 362030 6101020
-
Referring to Table 6 and Figure 10, it can be clearly seen that
the fatigue lines of the cold mix-1 2% RSC were atter than
the cold mix without cement. At a projected life to failure
(Nf ) of 106 load (pulse) cycles, the cold mix-1 2% RSC
could withstand a deformation of 59microstrains, which was
about twice the strain (deection) of the cold mix without
cement. Similarly, the number of cycles to failure (Nf ) of the
cold mix-1 2% RSC at a relatively low deection value of
100microstrains (4.84 104 cycles) was also much higher
than that of the cold mix-1 without cement. This indicates
that incorporating cement signicantly improves the
performance of cold mix-1, in particular at low strain levels;
for example, when acting as part of a well designed pavement
structure.
Figure 10 also shows that the fatigue performance of the cold
mix-1 2% RSC compared well with the 14mm asphalt
concrete 100 pen. hot mixes.25 However, without cement the
cold mixes were inferior. This is logical, in particular when
considering the air void values of the cold mixes which in
general were much higher than of the hot mixes.
The stiffness of the cold mixes with cement was a lot greater
than those of the hot mixes with which they were being
compared. However, as fatigue is only one of many
parameters that can be used to evaluate the performance of
asphalt mixes, it is possible that one mixture might exhibit
superior performance in one respect/area but be inferior in
others. This could only be explained by the lack of
repeatability of test results between different investigators.
11. CONCLUSIONS
The main conclusions drawn from this investigation are listed
here.
(a) Cold-lay, cold-mix emulsion mixtures (cold mixes), when
properly designed and at full curing, even without the
addition of cement, were comparable in stiffness to hot
mixtures (of equivalent grade bitumen).
(b) The addition of 12% cement by mass into cold mixes
signicantly improved the overall performance of the cold
mix; namely the stiffness (ITSM), creep resistance and
acceleration of strength gain. It did not, however, improve
the fatigue performance of the cold mix beyond that of the
hot mixes.
(c) When incorporating cement, the cold mix should be
compacted soon after mixing to maximise the results and to
avoid workability problems. If this is not possible the loose
mixture can be kept in a sealed container and compacted
after approximately 24 h (see Figure 4).
ACKNOWLEDGEMENT
The authors would like to thank: Eggborough Power Station
Management, Ferrybridge Power Station Management, Nynas
Ltd. UK, Tarmac Group-UK and Berryman Ltd. UK for their
materials supply and technical information.
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1000
100
10100 1000 10 000 100 000 1 000 000
Number of cycles to failure: Nf
Stra
in: m
icros
train
Cold mix-1Cold mix-1 +2% RSCAC
Figure 10. Cold mixes fatigue performance compared with hotmixes
Mixture Equation based on strain(Figure 9)
Equation based on numberof cycles to failure
ee atNf 10
6 cyclesNf (cycles) ate 100e
Cold mix-1 without cement e 3152:2 N0:3397
f Nf 2:0 1010e2:91 29 3:03 104
R2 0:9898 R2 0:9898
Cold mix-1 2% RSC e 679:37 N0:1774
f Nf 7:0 1015e5:58 59 4:84 104
R2 0:9899 R2 0:9899
14mm asphalt concrete e 1638:4 N0:2567
f Nf 3:0 1012e3:89 47 4:98 104
(100 pen. asphalt) R2 0:9991 R2 0:9991
Based on data presented in Figure 10, but with the strain plotted as the x-axis and the load cycles plotted as the y-axis.
Table 6. Fatigue line equations (exponential regression line/trend line) and coefficient of correlation (R2)
54 Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al.
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Transport 162 Issue TR1 A laboratory study on cold-mix, cold-lay emulsion mixtures Thanaya et al. 55
Abstract1. INTRODUCTION2. AGGREGATE GRADATIONS FOR THE MIXTURESEquation 1
3. MATERIALS USEDTable 1
4. MIXTURE DESIGNATIONFigure 1Table 2
5. MIXTURE PRODUCTION AND MECHANICAL PROPERTIES TESTED6. ITSM COMPARISON OF THE COLD MIX TO HOT MIXTURES (100 PEN. GRADE)Figure 2Table 3
7. EFFECT OF STORAGE TIME ON COLD MIXESFigure 3Figure 4Figure 5
8. OUTDOOR CURING OF THE COLD MIXESFigure 6
9. CREEP PERFORMANCEFigure 7Figure 8
10. FATIGUE PERFORMANCEFigure 9Table 4Table 5Figure 10Table 6
11. CONCLUSIONSACKNOWLEDGEMENTREFERENCESReference 1Reference 2Reference 3Reference 4Reference 5Reference 6Reference 7Reference 8Reference 9Reference 10Reference 11Reference 12Reference 13Reference 14Reference 15Reference 16Reference 17Reference 18Reference 19Reference 20Reference 21Reference 22Reference 23Reference 24Reference 25